The present invention relates to an agricultural vehicle, in particular, a tractor or a harvesting machine having a full-track design and a vehicle structure on each side of which a track roller unit is arranged that comprises a roller unit body to which a front deflector roll, a rear deflector roller and a plurality of yoke-type track rollers arranged therebetween are coupled and an endlessly closed track belt that wraps at least around the front deflector roller and the rear deflector roller along which the yoke-type track rollers roll.
It is known to equip agricultural vehicles such as tractors or harvesting machines with track roller units, which have a larger ground contact area than wheels and therefore have a greater transfer of tractive force with less slip and result in less ground compression. The designs of track-type tractors available on the market can be subdivided into pivot-steered four-track tractors, two-track tractors and half-track tractors.
Pivot-steered four-track tractors are usually found in the upper performance class and, due to their weight and size alone, are less suitable for transport tasks or lighter applications.
Two-track tractors have a track roller unit on each side of the vehicle, relative to the longitudinal axis of the vehicle, which is used to support the vehicle with respect to the ground (full-track design). The track roller units can usually rotate by a small angle about a vehicle transverse axis relative to the vehicle frame, wherein the vehicle frame is designed largely identical to that of a corresponding wheel tractor. Reference is made in this context to WO1998/40266, merely as an example. Two-track tractors must be designed to be nose-heavy, in order to obtain a uniform pressure distribution under the track roller units when the work operation utilizes tractive force.
A uniform distribution of pressure is desired in the case of pulling work, in particular, because this is the only way to utilize the full potential in terms of the transfer of tractive force and protecting the ground. The nose-heaviness required therefor is a disadvantage of conventional two-track tractors, however. The reason is that, in order to achieve the aforementioned advantages, there must be good coordination with the particular drag of the attached implement. However, since the drag can vary greatly between various implements and/or depending on the particular working conditions (e.g., soil properties, tilling depth, ground speed, etc.) in practical applications, an operator must compensate therefor by attaching a suitable front ballast on the vehicle before starting to drive. If tractive forces fluctuate during operation, it is no longer possible to apply ballast correctly. When the nose-heavy vehicle travels on asphalted roads, the tread bars of the track belt, which are usually made of rubber, are strongly braked and compressed upon entry into the latch, which results in high wear and associated increased operating costs of the vehicle.
Half-track tractors are usually converted wheel machines, in which, e.g., the rear wheels are replaced with track roller units. The altered transmission ratio results in a great reduction of the maximum speed. Track roller units also must be mounted on the front in the case of four-wheel drive tractors, due to the fixed transmission ratio between the front axle and the rear axle. In this context, reference is made to US 2007/0261898 A1 as an example. In addition to an altered suspension behavior of the overall vehicle, reduced operational reliability must be accepted.
WO 2013/113484 A2 makes known another concept for a half-track vehicle. In this case, the (rear) track roller unit is used as a drive element. The front axle (which is designed as a wheel axle) is used only to support vertical forces and as steering support. In this dual-axle vehicle, the track roller unit is suspended in the manner of a pendulum. The track therefore lifts up when tractive force is generated. This is avoided by controlled counterpressure of a hydraulic cylinder. The front axle is relieved by applying tractive force and by the force of the hydraulic cylinder. It is therefore possible to hold the pressure distribution under the track roller unit constant during field work. During road travel, however, the hydraulic force can be reversed and, therefore, the front deflector roller of the track roller unit can be relieved. The front axle (wheel axle) therefore assumes a greater load. This results in a more gentle entry by the track bars and therefore reduces wear. As compared to the classic two-track tractor, this concept provides advantages both on yielding ground and on hard ground. The front axle additionally reduces pitching oscillations and, therefore, increases driving comfort.
Since the vehicle described in WO 2013/113484 A2 has a non-driven front axle, the disadvantage of having to provide precise ballast exists in this case as well. A front axle load of zero would be ideal for tractive efficiency. In this case, there is a risk, however, that the vehicle could tip backward and therefore the front axle must remain loaded to a slight extent. This load cannot be used to transfer tractive force, however, and can compress the ground if the tires are too small. It would be conceivable also to drive the front axle. It would be too complex to adjust the advance that is required due to the different tractive force characteristics of wheel and track, however, especially since agricultural tractors are usually not equipped with an interaxle differential.
In addition to a hydraulic element on the track roller unit, the vehicle according to WO 2013/113484 A2 also has a high-value steering axle. Due to the low intended front axle load, the vehicle is equipped with differential steering in the rear axle, with which the Ackermann steering system of the front axle must be synchronized. In light of the above-described aspects, the overall complexity of the vehicle is high.